1. Field
The present invention relates generally to data communication, and more specifically to techniques for performing receiver spatial processing in a multiple-input multiple-output (MIMO) communication system.
2. Background
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission and is denoted as an (NT, NR) system. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS spatial channels, where NS≦min {NT, NR}. The NS spatial channels may be used to transmit up to NS independent data streams to achieve greater overall throughput. Spatial processing may or may not be performed by a transmitter and is performed by a receiver in order to transmit multiple data streams on the NS spatial channels.
The NS spatial channels may or may not be orthogonal to one another. Orthogonal spatial channels can only be obtained when both (1) the transmitter performs spatial processing with the proper steering vectors and (2) the receiver performs spatial processing with the proper spatial filter. The orthogonality of the spatial channels thus depends on (1) whether or not spatial processing was performed at the transmitter and (2) whether or not the spatial processing at both the transmitter and the receiver was successful in orthogonalizing the spatial channels. Each spatial channel is referred to as an “eigenmode” of the MIMO channel if the NS spatial channels are orthogonal to one another. In this case, NS data streams may be transmitted orthogonally on the NS eigenmodes. Performance is better when the spatial channels are orthogonal.
However, in a practical system, the NS spatial channels are usually not completely orthogonal to one another due to various reasons. For example, the spatial channels would not be orthogonal if (1) the transmitter has no knowledge of the MIMO channel or (2) the transmitter and/or the receiver have an imperfect estimate of the MIMO channel. If the spatial channels are not orthogonal, then each data stream will experience cross-talk from the other data streams at the receiver. The cross-talk acts as additive noise that degrades performance.
There is therefore a need in the art for techniques to mitigate the deleterious effects of cross-talk when transmitting data on multiple spatial channels in a MIMO system.